Problem: $ C = \left[\begin{array}{r}3 \\ 2 \\ 1\end{array}\right]$ $ A = \left[\begin{array}{rr}-1 & 3\end{array}\right]$ What is $ C A$ ?
Explanation: Because $ C$ has dimensions $(3\times1)$ and $ A$ has dimensions $(1\times2)$ , the answer matrix will have dimensions $(3\times2)$ $ C A = \left[\begin{array}{r}{3} \\ {2} \\ \color{gray}{1}\end{array}\right] \left[\begin{array}{rr}{-1} & \color{#DF0030}{3}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ C$ , with the corresponding elements in column $j$ of the second matrix, $ A$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ C$ with the first element in ${\text{column }1}$ of $ A$ , then multiply the second element in ${\text{row }1}$ of $ C$ with the second element in ${\text{column }1}$ of $ A$ , and so on. Add the products together. $ \left[\begin{array}{rr}{3}\cdot{-1} & ? \\ ? & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ C$ with the corresponding elements in ${\text{column }1}$ of $ A$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{-1} & ? \\ {2}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ C$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ A$ and add the products together. $ \left[\begin{array}{rr}{3}\cdot{-1} & {3}\cdot\color{#DF0030}{3} \\ {2}\cdot{-1} & ? \\ ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{3}\cdot{-1} & {3}\cdot\color{#DF0030}{3} \\ {2}\cdot{-1} & {2}\cdot\color{#DF0030}{3} \\ \color{gray}{1}\cdot{-1} & \color{gray}{1}\cdot\color{#DF0030}{3}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}-3 & 9 \\ -2 & 6 \\ -1 & 3\end{array}\right] $